I can't answer about passing the soy through the egg...
But I can tell you why I choose soy free food.
Soy is one of the largest GMO crops that means very shortly before harvest tons of roundup is sprayed per acre to kill weeds the GMO is done to make the soybeans roundup resistant. I don't want to eat food that was sprayed with roundup right before collection and then sent to me. In addition soy and the demand for soy is one of the agricultural product that is helping destroy the rainforest, Brazil is like the third largest grower of soy and China (unregulated) is first or second. Soy has many beneficial properties but mostly fermented soy it also has properties that enhances estrogen responses so lots of soy can create female problems.
I didn't go looking for a soy free feed. I went looking for a whole grain organic food and was happy when it turned out to also be soy free. I use Countryside Organics and I am happy with the product. My chickens have been on it for over a year now and I've done layers and meaties. I was happy with the results of both. My layers which are mixed breeds and not all high production. I have some reds but I also have some americauna mixes. They have about a 90% rate of lay during spring-fall and this winter about 70% and my meaties dressed out from 5-6+pounds.